An example of a conventional radio transmission system having a 1+0 configuration, i.e., having no redundant configuration of an active system and a standby system (which does not relates to inventions that are disclosed in known publications) is explained hereinafter with reference to FIG. 10. This system is an example in which modulation mode change control is possible. However, since the radio transmission line has no redundancy, no relief can be given against the fading by the switching between the active and standby transmission lines.
FIG. 10 shows the reception side of a conventional modulation mode switching type communication device. An opposed radio communication device has identical functions and performance, though the illustration is omitted. An antenna 31 receives one type of radio frequency and outputs the received radio frequency to a receiving circuit 32. The receiving circuit 32 extracts a radio frequency from the received signal a, converts the extracted radio frequency into an intermediate frequency, and outputs the IF signal b to a demodulation circuit 33. Further, the receiving circuit 32 detects the reception level that fluctuates in a fraction of a second due to effects such as rainfall and fading in an analog fashion, and outputs a reception monitoring signal c that is obtained by performing an A/D (analog/digital) conversion without performing any other processing including averaging to a control circuit 35.
If the reception level is averaged, a delay occurs before the determination of the modulation mode is completed. As a result, in a radio system in which continuous signals are transmitted, fluctuations in the reception level caused by the effects such as rainfall and fading cannot be followed without causing delay, and consequently increasing the possibility that bit errors could occur. Therefore, the averaging of the reception level is not performed.
The reception monitoring signal c input to the control circuit 35 is compared with a lower-limit threshold and an upper-limit threshold. As a result of these comparisons, if the reception monitoring signal c is equal to or lower than the lower-limit threshold, the control circuit 35 changes the modulation multi-valued number to a low multi-value so that the system gain is led to the increasing direction. Further, if the reception monitoring signal c is larger than the upper-limit threshold, it changes the modulation multi-valued number to a high multi-value so that the system gain is led to the decreasing direction.
The instruction for this modulation mode change is output as modulation mode switching control information e1 by the control circuit 35 based on the above-described comparison determination result between the reception level and the thresholds, and also transmitted to an opposed station (not shown). In this way, after the matching of the modulation mode switching control information e1 between the opposed stations is confirmed, the modulation mode of the demodulation circuit 33 of the own station and that of a modulation circuit (not show) in the transmission-side device 2A are changed.
The demodulation circuit 33 reproduces a baseband signal d by demodulating the IF signal b of the receiving circuit 32. This baseband signal d is output after being converted into a bipolar signal, an optical signal, and/or the like in a baseband signal processing circuit 34. The demodulation is performed in a demodulation mode corresponding to the modulation mode based on the above-described modulation mode switching control information e.
Further, as a modulation mode switching communication technique of this kind, other techniques are known in which: the line quality is monitored in the demodulation side: the modulation mode control is switched in synchronization with a frame pulse such that the modulation multi-valued number is decreased when the line quality is deteriorated due to the effects such as rainfall attenuation and fading, whereas the modulation multi-valued number is increased when the line quality is improved; and by doing so, the opposed modulation and demodulation are changed to the same modulation mode so that the signal passage can be realized (for example, see Patent document 1).
Further, other techniques are also known in which, when different frequencies are used between the upward transmission line and the downward transmission and thus there is an asymmetry between them as in the case of the FDD (Frequency Division Duplex) system, the state of the transmission line having a broader transmission band between those transmission lines is preferentially selected, and the modulation mode is determined according to the state of this transmission line (for example, see Patent document 2).    [Patent Document 1]    Japanese Unexamined Patent Application Publication No. 2005-223835 (pages 2, and 10 to 12, and FIG. 1)    [Patent Document 2]    Japanese Patent No. 3898192 (pages 3 and 4, and FIG. 1)